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Modern pollen and vegetation relationships in Mar Chiquita coastal lagoon area, southeastern Pampa grasslands, Argentina
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Review of Palaeobotany and Palynology 126 (2003) 183^195 www.elsevier.com/locate/revpalbo
Modern pollen and vegetation relationships in Mar Chiquita coastal lagoon area, southeastern Pampa grasslands, Argentina Silvina Stutz a; , Aldo R. Prieto a;b a
Laboratorio de Paleoecolog|¤a y Palinolog|¤a, Depto. de Biolog|¤a, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, 7600 Mar del Plata, Argentina b CONICET, Argentina Received 21 August 2002; received in revised form 11 March 2003; accepted 20 May 2003
Abstract The relationships between vegetation and modern pollen are examined along a coast to inland gradient of plant communities in the Mar Chiquita coastal lagoon area, southeastern Pampa grasslands, Argentina. Local psammophytic, halophytic and freshwater plant communities that develop on coastal barriers, salt marshes and in shallow lakes respectively, were sampled for pollen analysis. The distribution of these communities can be related to specific environmental parameters such as topography, water table depth, substrate type and salinity. Samples were classified using Cluster Analysis and were ordinated using Detrended Correspondence Analysis (DCA). Results indicate that the plant communities sampled have distinct pollen assemblages: the psammophytic community is characterized by Poaceae, Cyperaceae and maximum values of psammophytic types; the halophytic community is characterized by maximum values of Chenopodiaceae (Salicornia ambigua) and the freshwater community is characterized by maximum values of both Cyperaceae and hydrophytic types, and by Poaceae. Subsequently, fossil data from a core from Mar Chiquita coastal lagoon were compared to the modern data set in search of similarities between fossil and modern pollen spectra. A composite DCA revealed that the fossil pollen assemblages have modern counterparts, permitting to interpret the vegetation history of the area in greater detail than previously possible. - 2003 Published by Elsevier B.V. Keywords: modern pollen; vegetation; environmental parameters gradient; Pampa grasslands; Holocene; vegetation history; Argentina
1. Introduction Along the coast of the Pampa grasslands, Ar-
* Corresponding author. Fax: +54-223-4753550. E-mail address: [email protected] (S. Stutz).
gentina, several paleoecological studies have demonstrated a complex history of Holocene vegetational and environmental changes in response to postglacial sea-level £uctuations, leading ultimately to modern patterns of plant communities (Prieto, 1996; Quattrocchio et al., 1998; Prieto et al., 1999; Stutz et al., 1999). Particularly, in the area of the Mar Chiquita coastal lagoon, changes
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in plant communities have been reconstructed using pollen analysis by Nieto and D’Antoni (1985), Prieto (1993), Stutz (2000) and Stutz et al. (2002). Holocene sea-level £uctuations a¡ected the whole Argentine coastal zone, from Rio de La Plata to Southern Patagonia (Isla, 1989, 1998). Because of their own morphology, the nature and extent of sea-level £uctuations a¡ected every site in particular ways, and past changes in plant communities re£ect the local response to these £uctuations. The vegetation of the Mar Chiquita coastal lagoon area is characterized by several plant communities : psammophytic, halophytic and freshwater communities and isolated woodlands. These communities can be related to speci¢c environmental parameters, such as topography, water table depth, substrate type and salinity, which change along a few kilometers-long gradient from the Atlantic coast towards inland. The existence of well-marked landforms, such as beaches, coastal lagoons, salt marshes, barriers, and shallow lakes (lagunas) is related to the Holocene transgressive^regressive sea-level history (Schnack et al., 1982; Fasano, 1991; Violante, 1992). According to Janssen (1984), when local plant communities can be reconstructed in a spatial arrangement of pollen assemblages, and when these same groups are identi¢ed within a temporal (stratigraphic) arrangement of pollen groups in sediment cores, more accurate interpretations of the nature of local vegetation succession can be made. Therefore the spatial arrangement of plant communities in the Mar Chiquita coastal lagoon area provides an excellent potential for modern analogs that can be used to reconstruct the vegetational succession related to sea-level £uctuations. The principal aim of this paper is to establish the relationship between plant communities along a spatial gradient and their respective modern pollen assemblages, in order to obtain a more objective interpretation of fossil pollen assemblages. First pollen spectra of surface sediment samples are presented and numerical methods are used to classify them. Second, a numerical analysis is carried out to study analogies between modern and fossil pollen spectra.
2. Study area, physical setting and vegetation The study area is located in the southeastern part of the Pampa grasslands (Fig. 1). It has an extension of 300 km2 that encompasses the Mar Chiquita coastal lagoon (37‡43PS; 57‡24PW) and Laguna Hinojales (37‡34PS; 57‡27PW). The Mar Chiquita coastal lagoon has a north-south orientation and it is connected to the Atlantic Ocean by a narrow inlet, subject to natural and maninduced changes. It has an area of 46 km2 , with a maximum length of 25 km and a maximum width of 5 km. Laguna Hinojales has an area of 5 km2 and is located west of the Mar Chiquita coastal lagoon. The climate is subhumid^humid (Burgos and Vidal, 1951), with a mean annual temperature of 13.7‡C and a mean annual rainfall of 912 mm, mostly occurring in summer (Servicio Meteorolo¤gico Nacional, 1951^1980). A variety of landforms are present in the study area; some date back to late Pleistocene times, while others are related to the Holocene transgressive-regressive sea-level £uctuations or are still in evolution (Schnack et al., 1982; Fasano, 1991; Violante, 1992) (Fig. 1). Above the 5 m asl, the Pampa plain extends across the western and southern part of the study area, showing an extremely low topographic gradient. Its origin is related to dry conditions that prevailed during the late Pleistocene, shown by the presence of numerous shallow de£ation basins, today in many cases occupied by permanent freshwater (e.g. lagunas Hinojales and Nahuel Ruca¤). Bordering these depressions, half-moon shape silty dunes have developed reaching altitudes of 10^15 m above the surrounding plain (about 25 m asl). Their origin has been associated to the development of a de£ation^accumulation relief under arid conditions during the late Pleistocene. An elongated feature extending from near Laguna Nahuel Ruca¤ to the northeast and parallel to the coastline, separates the Pampa plain from landforms to the east. This Pleistocene barrier, with a maximum altitude of 10 m asl, is interpreted as an ancient sandy barrier related to the late Pleistocene marine transgression and together with the westernmost littoral shell ridge, mark the western and southern limit
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Fig. 1. Map showing the location of the Mar Chiquita coastal lagoon area, landforms, location of surface pollen samples and the site of La Lagunita core.
of the Holocene transgression (Fasano, 1991). Below 5 m asl, marginal £ats have developed ¢lled with Holocene sediments, that corresponds to the Mar Chiquita coastal lagoon and the surrounding salt marsh. South of Laguna de Sotelo, towards the north and also locally emerging within the marginal £at, there are littoral shell ridges. They constitute old
spits, bars and storm-beach accumulations formed during the Holocene transgressive^regressive sealevel £uctuations. The area is separated from the Atlantic Ocean by a coastal barrier, which corresponds to sandy dunes and adjacent beaches. This landform, with a maximum altitude of 30^40 m asl, extends from the Mar Chiquita lagoon inlet to the north increasing in height and width. The
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Fig. 2. Vegetation map of Mar Chiquita coastal lagoon area, modify after Isacch (2001). The location of surface pollen samples is shown.
coastal barrier represents the eolian cycle associated with the transgressive-regressive sea-level £uctuations, and continues in evolution today. Very distinctive plant communities develop on the above described landforms (Vervoorst, 1967; Stutz, 2001) (Fig. 2). Vegetation of the Pampa plain is characterized by a grass steppe dominated by grasses of the genera Piptochaetium, Stipa, Bo-
thriochloa and Aristida (Cabrera, 1976; Leo¤n, 1991). In more humid areas the vegetation is characterized by sedges (Cyperus, Carex, Schoenoplectus, Eleocharis), grasses (Paspalum, Stipa, Panicum, Polypogon, Glyceria, Paspalidium), several species of Juncus and herbaceous dicots (Eryngium, Aster, Daucus, Phyla). In the depressions, the shallow lakes plant communities (freshwater
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community) are characterized by aquatic plants (emergent, £oating-leaf and submergent). In the deepest zones the Myriophyllum elatinoides and Ceratophyllum demersum develop, while at the lake margins, Azolla ¢liculoides, Ricciocarpus natans and Lemna valdiviana are common. The dominant shore species is Schoenoplectus californicus, accompanied with Zizaniopsis bonariensis and Solanum glaucophyllum. Other common emergent species such as Senecio bonariensis, Ranunculus bonariensis, R. apiifolius, Alternanthera philloxeroides, Buddlegia elegans, Hydrocotyle bonariensis, H. ranunculoides, Polygonum punctatum, Eryngium sp. and other sedges such as Carex, Eleocharis and Cyperus are commonly found. On silty dunes, monospeci¢c woodlands of Celtis tala develop. Isolated individuals of Celtis tala also occur on the Pleistocene barrier and on the westernmost littoral shell ridge, as a consequence of bird seed dispersion. These woodlands constitute the southernmost distribution of the Tala district, which extends from the south of Entre R|¤os and Santa Fe provinces to the Mar Chiquita coastal lagoon as a narrow belt forming the eastern limit of Pampa grasslands (Cabrera, 1976). The vegetation of the Pampa plain is highly modi¢ed by farming activities that transformed the area into crop land and grazing ¢elds, mainly growing wheat, maize, soybean and arable pastures. On the marginal £at, the salt marsh of Mar Chiquita lagoon is characterized by a halophytic community (‘espartillar’) dominated by Spartina densi£ora. In the seasonally or permanently £ooded zone, a belt of Spartina densi£ora occurs bordering the lagoon accompanied by the mud£at colonist Salicornia ambigua. Above this zone, Distichlis spicata and D. scoparia codominate, accompanied by Atriplex montevidensis, Spartina alterni£ora, Grindelia discoidea, Sida leprosa, Limonium brasiliense, among the most important. In the highest zone, only a¡ected by episodic £ooding, patches of Juncus acutus occur. Also Paspalum vaginatum, Ambrosia tenuifolia, Hydrocotyle bonariensis, Scirpus cernuus, Apium sellowianum and Samolus valerandi are common. Bordering the freshwater canals that £ow into Chico creek or into the lagoon the sedges Schoenoplectus chilensis and Eleocharis bonariensis are frequent, as-
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sociated with other common freshwater species. The emergent littoral shell ridges have been transformed into grazing ¢elds and the natural vegetation is highly modi¢ed. The vegetation of the coastal barrier is mainly characterized by psammophytic species of Poaceae, Cyperaceae and Asteraceae families which grow on sandy soils (psammophytic community), and is distributed according to the morphology of the dunes. Near the beach and on recently formed dunes, patches of Spartina coarctata develop. Calycera crassifolia, Senecio crassi£orus and the adventitious Cakile maritima are the most important associated species. Mobile dunes are characterized by the pioneer grass Panicum racemosum. At the crests and slopes of low dunes as well as in lightly humid interdune depressions, Androtrichium tryginum and Tessaria absinthioides are dominant, frequently associated with Cortaderia dioica. In the interdune depressions where the water table is near the surface or outcropping, Typha angustifolia and T. latifolia, the sedges Schoenoplectus californicus, S. maritimus, Carex extensa, Eleocharis montevidensis and several species of Juncus prevail. On ¢xed dunes Adesmia incana and Poa lanuginosa are dominant, accompanied by Poa barrosiana, Hydrocotyle bonariensis, Margyricarpus pinnatus, Solidago chilensis, Oenothera mollissima, Polygala cyparissias, Senecio crassi£orus, Ambrosia tenuifolia, Baccharis juncea, B. microcephala, Gnaphalium cheiranthifolium, Daucus montevidensis, Androtrichium tryginum, and the adventitious Centaurium pulchellum, Blackstonia perfoliata, Medicago lupulina, Melilotus indicus and M. albus. Many exotic trees were introduced in the area at the beginning of the 20th century for public parks and gardens (e.g. Populus alba, Betula pendula, Casuarina sp., Juglans nigra, Araucaria araucana and A. angustifolia). On the coastal barrier plantations of Pinus, Cedrus, Cupressus, Eucalyptus and Acacia longifolia are common.
3. Materials and methods Sampling sites were chosen according to the major plant communities that develop on the different landforms. In the Pampa plain, because
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its vegetation is highly modi¢ed (Fig. 2), only shallow lakes freshwater communities were sampled. A total of 24 surface sediment samples were collected (Figs. 1 and 2). Seven samples from the freshwater community of Laguna Hinojales and 11 samples from the halophytic community of the salt marsh of Mar Chiquita lagoon were collected using a Dachnovsky core sampler and sub-sampling the top 2 cm of each core. Six samples were collected at the psammophytic community of the coastal barrier from the upper soil to a depth of 2^5 cm with a spatula. Sediment samples were processed according to standard palynological techniques including warm KOH, HCl, heavy-liquid separation with ZnCl2 , HF and acetolysis (Gray, 1965; Faegri and Iversen, 1989). Five Lycopodium clavatum tablets were added to every weighted sample before treatment to estimate representative pollen sums (Bianchi and D’Antoni, 1986). Total pollen sums varied from 230 to 1420 grains. Identi¢cations and counting were performed using the pollen reference collection of the Laboratory of Palaeoecology and Palynology, University of Mar del Plata. Additional pollen data from ¢ve surface sediment samples from the bottom of the Mar Chiquita coastal lagoon (Nieto, 1985) were considered. In the pollen diagrams some genera or species were grouped into family categories. Asteraceae pollen includes Ambrosia/Xantium, undi¡erentiated Asteroideae and Mutisieae. Apiaceae pollen includes Hydrocotyle, Apium, Eryngium, Daucus and undi¡erentiated Apiaceae. Other species were grouped according to growth habitat. Psammophytic types include herbs taxa that develop on sandy soils: Fagara sp., Adesmia incana, Calycera crassifolia, Oenothera mollissima, Blackstonia perfoliata, Centaurium pulchellum, Polygala cyparissias, Margyricarpus pinnatus and Caryophyllaceae; and hydrophytic types include emergent and submergent aquatic taxa: Alternanthera philoxeroides, Myriophyllum elatinoides, Ranunculus spp. and Solanum glaucophyllum. Extra-regional types and exotic arboreal taxa are not included in the pollen sums for percentage calculation. Pollen diagrams were plotted using TILIAGRAPH 1.25 (Grimm, 1991). Surface pollen samples were classi¢ed into
groups using Cluster Analysis (CA) and were ordinated by Detrended Correspondence Analysis (DCA; DECORANA). Statistical analyses were performed to determine whether the plant communities are associated with characteristic pollen spectra from which the parent vegetation can be identi¢ed. Only pollen taxa present at v 2% in at least three samples were included. Brassicaceae were excluded due to an overrepresentation in sample 4, which corresponds to the adventitious Cakile maritima, that would have distorted the statistical analyses. The analyses were performed on recalculated percentages after all modi¢cations were made. The ¢nal 12 pollen types selected were: Poaceae, Cyperaceae, Chenopodiaceae, psammophytic types, hydrophytic types, Asteraceae subf. Cichorioideae, Asteraceae, Celtis, Plantago, Typha, Papilionoideae and Apiaceae. Both analyses were performed using TILIA 1.12 (Grimm, 1991).
4. Results 4.1. Surface pollen diagram (Fig. 3) The zonation shown in this ¢gure is based on the classi¢cation of the sites at the time of sample collection (Figs. 1 and 2). The visual inspection of this ¢gure indicates the di¡erent plant communities/sites are distinguishable on the basis of their pollen assemblages. Samples from the psammophytic community of the coastal barrier (1^6) are characterized by variable values of Poaceae (12^68%) and Cyperaceae (6^57%) and maximum values of the psammophytic types (up to 6.5%). Samples from the halophytic community of the salt marsh (7^16) are characterized by maximum values of Chenopodiaceae (Salicornia ambigua) (55^80%) while the samples from the bottom of the Mar Chiquita coastal lagoon (17^22) are characterized by Chenopodiaceae (16^41%), Poaceae (10^34%), Asteraceae (14^30%) and Cyperaceae (6^16%). The samples from the freshwater community of Laguna Hinojales (23^29) are characterized by maximum values of both Cyperaceae (20^70%) and hydrophytic types (up to 5%), and by Poaceae (16^49%).
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PALBO 2539 18-9-03 Fig. 3. Percentage pollen diagram of 29 surface samples, arranged after sampling sites (plant community/landform).
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water community samples (cluster 3). Samples 15 and 16, also collected in the ‘espartillar’ but far from the lagoon’s shore, are grouped with coastal lagoon samples because of their low values of Chenopodiaceae. Samples 2 and 3 of the psammophytic community and sample 27 of the freshwater community are included in cluster 2 and became separated at the third level of division. These samples have the maximum values of Poaceae (up to 70%). The former two were collected near populations of Cortaderia dioica, and the latter was collected far from the Laguna Hinojales shore. Although in cluster 3, the samples of the psammophytic and the freshwater communities are mixed, at a third level of division the psammophytic community samples, except sample 1, are distinguished from the freshwater community samples, and at the fourth level the freshwater community samples which have maximum values of Cyperaceae (50^70%) and of hydrophytic types (up to 5%) (samples 23, 24, 28 and 29) became separated. 4.3. DCA (Fig. 5)
Fig. 4. CA of surface pollen samples. Samples are named according to sampling site: PS: psammophytic community; FW: freshwater community; HC: halophytic community and CL: coastal lagoon.
4.2. CA (Fig. 4) The ¢rst division clearly separates the most of the halophytic community samples (cluster 1) from all others. These samples have maximum values of Chenopodiaceae (55^80%) and were collected in the halophytic community (‘espartillar’) near the coast of Mar Chiquita lagoon, where Salicornia ambigua mainly develops. The second division separates all the coastal lagoon samples and samples 15 and 16 of de halophytic community (cluster 2), which have lower values of Chenopodiaceae (21^38%), from the most of both, psammophytic community samples and fresh-
The DCA ordination of the samples broadly equates with the plant communities/sites recognized in the ¢eld. The ¢rst axis (eigenvalue 0.35) contrasts Chenopodiaceae, Plantago and Asteraceae subf. Cichorioideae with Cyperaceae, psammophytic types, Apiaceae, Typha, hydrophytic types and Papilionoideae. It separates the halophytic community samples and the coastal lagoon samples from the psammophytic community samples and the freshwater community samples. Poaceae, Celtis and Asteraceae against Chenopodiaceae di¡erentiate only slightly between the coastal lagoon samples and the halophytic community samples. The second axis (eigenvalue 0.04) contrasts psammophytic types, Apiaceae, Celtis and Poaceae with hydrophytic types and Papilionoideae. It separates the psammophytic community samples from the freshwater community samples. Sample ordination suggests that along axis 1, plant communities change from freshwater to brackish conditions, re£ecting the increase of salinity in the substrate. Sites progressively separated along axis 2 re£ect the e¡ect of water table
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Fig. 5. DCA biplot of axes 1 and 2. (a) Sample scores with memberships of the respective sampling sites. (b) Pollen variable scores.
depth and substrate type in plant communities distribution.
5. Discussion When comparing modern pollen assemblages and vegetation, the three plant communities under consideration are easily distinguishable by their pollen spectra. Several interacting factors determine the degree in which each plant community is re£ected by the
pollen spectra. Among the most important are pollen production and dispersal characteristics of the species, the means of dispersion, insect or wind, the local-scale patterns of the species distribution at each community and the representativeness of local and extra-local pollen rain. Considering the three plant communities, the clearest signature is detected in the halophytic community re£ected by the samples from the salt marsh and from the coastal lagoon. These groups of samples are well discriminated in the CA and by the ¢rst axis of the DCA. The princi-
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pal pollen type associated with these groups is Chenopodiaceae. Samples from the halophytic community show an over-representation of this taxon (mainly representing Salicornia ambigua), even though the dominant species is Spartina densi£ora (Poaceae). This is probably attributable to a di¡erence in their pollen dispersion^deposition. Both species are abundant pollen producers and wind-pollinated. Spartina bears its in£orescence to 1^1.5 m instead of Salicornia that only can reach 0.80 m high. This di¡erence in height probably limits the uplift of Chenopodiaceae pollen into the turbulent upper air layer, greatly restricting its pollen dispersion and favoring in situ deposition. Samples 15 and 16 were grouped with the samples from the coastal lagoon. They were collected in the middle zone of the salt marsh, far from the coast of the lagoon where Salicorna ambigua declines in number and the co-dominant species are Distichlis spicata, D. scoparia and Spartina alterni£ora (Poaceae). Chenopodiaceae is well represented when present locally, but pollen values decline rapidly with distance from the source, or when its presence is only a small proportion of the vegetation. Samples from the coastal lagoon, mainly characterized by Chenopodiaceae, Poaceae and Asteraceae, also represent the halophytic community. Chenopodiaceae percentages are less than 50% and similar to those of Poaceae. These samples form a tight group in the CA and became slightly separated from the halophytic community samples by the ¢rst axis of the DCA. In addition to Chenopodiaceae pollen taxa associated with these samples are Poaceae, Plantago, Celtis, Asteraceae, and Asteraceae subf. Cichorioideae, emphasizing the role of the lagoon as a pollen catchment site. The lagoon not only collects pollen from the surrounding low salt marsh, but also pollen from species of the middle and the high salt marsh, such as Distichlis and Juncus acutus (Monocotyledonae). On the contrary, samples from psammophytic and freshwater communities show some overlap in pollen assemblages, as indicated by the ¢rst axis of the DCA, the CA groups and the pollen percentages diagram, where both groups of samples are similar because their comparable Cyperaceae and Poaceae values. Due to the di⁄culty of
a genus or species level identi¢cation, both Poaceae and Cyperaceae include psammophytic species (Androtrichium tryginum, Schoenoplectus maritimus, Cortaderia dioica, Poa lanuginosa, P. barrosiana, Spartina coarctata and Panicum racemosum) as well as aquatic species (Schoenoplectus californicus, Carex sp., Eleocharis sp., Cyperus sp. and Zizaniopsis bonariensis). Another point to be considered is that both plant communities share some species, like the sedges that also occur at interdune depressions (Schoenoplectus, Carex and Eleocharis). The separation between these communities is possible by few taxa poorly represented in the pollen spectra : the speci¢c psammophytic and hydrophytic types, as it is shown in the second axis of the DCA. Under-representation of these types in the pollen spectra is due to their low plant frequency and that they are insect-pollinated producing relatively low quantities of pollen grains. Their importance for the characterization of these communities is clearly shown by the second axis of DCA, where they are correlated with both, the substrate type and water table depth. Aquatic species included in hydrophytic types develop on the silty-clay substrates of the shallow lakes, where the water table is near the surface or outcropping, while psammophytic species, included in psammophytic types, develop on the sandy substrates of the dunes, where the water table is deeper.
6. Application to interpret fossil pollen assemblages To illustrate the applicability of the modern pollen assemblages in the interpretation of plant community evolution, 41 fossil samples from La Lagunita core (Figs. 1 and 6) were combined with the 29 modern surface samples using DCA (Fig. 7). The fossil pollen diagram was divided in three zones and two subzones by CA Stratigraphical Constrained (Stutz, 2000). Zone LL1 (ca. 6200^ 5130 yr BP) is dominated by Poaceae accompanied by Chenopodiaceae and presents constant values of psammophytic types. Zone LL2 (5130^ 2570 yr BP) is characterized by an increase of Chenopodiaceae and a decrease of Poaceae. At
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Fig. 6. Pollen percentage diagram of La Lagunita core. Pollen zones according to CA, depth, radiocarbon dates and estimated ages are indicated.
the top of this zone (LL2b) psammophytic types decrease. Zone LL3 (2570 yr BP-present) is characterized by the highest values of Chenopodiaceae. DCA ordination of modern and fossil samples shows that except for the pre-5130 yr BP interval, the fossil samples have modern counterparts, thus the gradual transition and replacement of plant communities can be interpreted on the basis of modern plant communities. Zone LL1 has no analogs in the modern spectra. The relatively high Poaceae values combined with high Chenopodiaceae suggests a mosaic of psammophytic and halophytic communities developing on sandy barrier and salt marsh/coastal lagoon
environment that have no modern counterpart. Subzones LL2a and LL2b form a continuum that is partially analogous to coastal lagoon and salt marsh halophytic communities. The whole zone LL2 shows the gradual replacement from the psammophytic^halophytic mosaic to the halophytic community, which increases its development until the modern state, as shown by zone LL3. Zone LL3 falls within the halophytic salt marsh community group, suggesting that the installation of this community began ca. 2500 yr BP. The continuous presence of Celtis tala in the fossil spectra, with percentage values similar to those of the modern spectra, suggests that these
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Fig. 7. DCA of combined surface and fossil pollen samples.
woodlands have been present in the area since Middle Holocene.
7. Conclusions Di¡erent plant communities in Mar Chiquita coastal lagoon area are clearly distinguished by their pollen spectra. These communities, determined by environmental parameters such as topography, salinity, substrate type and water table depth, re£ect the few kilometers-long gradient from coast to inland. Furthermore, the precision with which modern pollen spectra can de¢ne plant community boundaries at a small spatial scale, greatly increases the accuracy in the reconstruction of past vegetation. Such precision is desirable for paleoecological reconstructions dealing with changes in local plant communities related to sea-level £uctuations. In addition, this study provided valuable analogs to interpret Holocene vegetational history (e.g. Stutz, 2000; Stutz et al., 2002), related to sea-level £uctuations in other sites on the coast of the Pampa grasslands.
Acknowledgements This research was funded through grants from
the Universidad Nacional de Mar del Plata (15/ E138), CONICET (PIP N‡0418/98) and Agencia Nacional de Promocio¤n Cient|¤¢ca y Tecnolo¤gica FONCYT (PICT 07-06477). Thanks are due to V. Mancini and F.I. Isla for their helpful comments; Claudio Perez for his help in the ¢eld and for improvement of the English text; R and M. Arbelaiz and A. Romano for giving permission to work in their properties ; M. Tonello for drafting Fig. 1. We are grateful to H. Hooghiemstra and to an anonymous reviewer for critical and valuable suggestions. References Bianchi, M.M., D’Antoni, H.L., 1986. Depositacio¤n del polen actual en los alrededores de Sierra de Los Padres (Pcia. de Buenos Aires). IV Congreso Argentino de Paleontolog|¤a y Bioestratigraf|¤a. Ape¤ndice de las Actas, 16^27. Mendoza, Argentina. Burgos, J., Vidal, A.L., 1951. Los climas de la Repu¤blica Argentina segu¤n la nueva clasi¢cacio¤n de Thornthwaite. Meteoros 1, 1^32. Cabrera, A.L., 1976. Regiones Fitogeogra¤¢cas Argentinas. Enciclopedia Argentina de Agricultura y Jardiner|¤a. Editorial ACME. T.II. Buenos Aires. 85 pp. Faegri, K., Iversen, J., 1989. Texbook of pollen analysis. In: Faegri, K., Kalana, P.E. and Krzywinski, K. (Eds.), IV Edition. John Willey and Sons, New York, 328 pp. Fasano, J.L., 1991. Geolog|¤a y geomorfolog|¤a. Regio¤n III. Faro Querand|¤ - Mar de Cobo. Provincia de Buenos Aires.
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S. Stutz, A.R. Prieto / Review of Palaeobotany and Palynology 126 (2003) 183^195 Informe Final. Convenio de Cooperacio¤n Horizontal. CFI y UNMdP. 118 pp. Unpublished. Gray, J., 1965. Palynological Techniques. W.H. Freeman, San Fransisco, CA, 471^587. Grimm, E., 1991. Tilia Software. Illinois State Museum. Research and Collection Center. Spring¢eld, IL. Isacch, J.P., 2001. Mapa de vegetacio¤n de la reserva Mar Chiquita y a¤reas circundantes. In: Iribarne, O. (Ed.), Reserva de Bio¤sfera Mar Chiquita: Caracter|¤sticas f|¤sicas, biolo¤gicas y ecolo¤gicas. Editorial Martin, Mar del Plata, pp. 79^81. Isla, F.I., 1989. Holocene sea level £uctuation in the southern hemisphere. Quat. Sci. Rev. 8, 359^368. Isla, F.I., 1998. Holocene coastal evolution in Buenos Aires Province, Argentina. Quat. S. Am. Antarct. Penins. 8, 297^ 321. Janssen, C.R., 1984. Modern pollen assemblages and vegetation in the Myrtle Lake Peatland, Minnesota. Ecol. Monogr. 54, 213^252. Leo¤n, R.J.C., 1991. Setting and vegetation. In: Coupland, R.T. (Ed.), Natural Grassland: Introduction and Western Hemisphere. Elsevier, Amsterdam. Nieto, M.A., 1985. Informe ¢nal Beca de perfeccionamiento Comisio¤n de Investigaciones Cient|¤¢cas de la Provincia de Buenos Aires. Unpublished. Nieto, M.A., D’Antoni, H.L., 1985. Pollen analysis of sediments of the Atlantic shore at Mar Chiquita (Buenos Aires Province, Argentina). Zentralbl. Geol. Pala«eontol. I, 1731^ 1738. Prieto, A.R., 1993. Palinolog|¤a de sedimentos lagunares del Holoceno en la Provincia de Buenos Aires: In: Boltovskoy, A., Lo¤pez, H.L. (Eds.), Una Revisio¤n. Conferencias de Limnolog|¤a. pp. 203^216. Prieto, A.R., 1996. Late Quaternary vegetational and climatic changes in the Pampa Grassland of Argentina. Quat. Res. 45, 73^88. Prieto, A.R., Lorscheitter, M.L., Stutz, S., 1999. Holocene vegetation changes in relation to the coastal evolution in Buenos Aires Province (Argentina) and Rio Grande do Sul
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(Brazil). Anais, VII Congreso da ABEQUA. Porto Seguro. viiabequa_zco027.pdf. Quattrocchio, M.E., Grill, S.C., Zaballa, C.A., 1998. Chronoestratigraphic and Palynozone chronosequences charts of Naposta¤ Grande Creek, Southeastern Buenos Aires Province, Argentine. Quat. S. Am. Antart. Penins. 7, 111^133. Schnack, E.J., Fasano, J.L., Isla, F.I., 1982. The evolution of Mar Chiquita Lagoon coast, Buenos Aires Province, Argentina. Holocene Sea Level Fluctuations, Magnitude and Causes. I.G.P.C., 143^155. 61 Annual Meeting, South Carolina. Servicio Meteorolo¤gico Nacional, 1951^1980. Estad|¤sticas clima¤ticas, Buenos Aires. Stutz, S., 2000. Historia de la vegetacio¤n en el sudeste de la provincia de Buenos Aires durante el u¤ltimo ciclo trasngresivo-regresivo del Holoceno. Doctoral Thesis. University of Mar del Plata. 128 pp, unplublished. Stutz, S. 2001. Vegetacio¤n del a¤rea de la laguna Mar Chiquita. In Iribarne, O. (Ed.), Reserva de Bio¤sfera Mar Chiquita: Caracter|¤sticas f|¤sicas, biolo¤gicas y ecolo¤gicas. Editorial Martin, Mar del Plata, pp. 75^78. Stutz, S., Prieto, A.R., Isla, F.I., 1999. Cambios de la vegetacio¤n durante el Holoceno en el SE bonaerense: Ana¤lisis pol|¤nico del paleoestuario del Arroyo La Ballenera, Provincia de Buenos Aires. Asoc. Paleontol. Argent. Publ. Espec. 6, 65^69. Stutz, S., Prieto, A.R., Isla, F.I., 2002. Historia de la vegetacio¤n del Holoceno de la laguna Hinojales, sudeste de la provincia de Buenos Aires, Argentina. Ameghiniana 39, 85^94. Vervoorst, F., 1967. La vegetacio¤n de la Repu¤blica Argentina. VII Las comunidades vegetales de la depresio¤n del Salado (Provincia de Buenos Aires). INTA, Serie Fitogeogra¤¢ca 7. Buenos Aires, 262 pp. Violante, R.A., 1992. Ambientes sedimentarios asociados a un sistema de barrera litoral del Holoceno en la llanura costera al sur de Villa Gesell, Provincia de Buenos Aires. Rev. Asoc. Geol. Argent. 47, 201^214.
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Modern pollen and vegetation relationships in Mar Chiquita coastal lagoon area, southeastern Pampa grasslands, Argentina Silvina Stutz a; , Aldo R. Prieto a;b a
Laboratorio de Paleoecolog|¤a y Palinolog|¤a, Depto. de Biolog|¤a, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, 7600 Mar del Plata, Argentina b CONICET, Argentina Received 21 August 2002; received in revised form 11 March 2003; accepted 20 May 2003
Abstract The relationships between vegetation and modern pollen are examined along a coast to inland gradient of plant communities in the Mar Chiquita coastal lagoon area, southeastern Pampa grasslands, Argentina. Local psammophytic, halophytic and freshwater plant communities that develop on coastal barriers, salt marshes and in shallow lakes respectively, were sampled for pollen analysis. The distribution of these communities can be related to specific environmental parameters such as topography, water table depth, substrate type and salinity. Samples were classified using Cluster Analysis and were ordinated using Detrended Correspondence Analysis (DCA). Results indicate that the plant communities sampled have distinct pollen assemblages: the psammophytic community is characterized by Poaceae, Cyperaceae and maximum values of psammophytic types; the halophytic community is characterized by maximum values of Chenopodiaceae (Salicornia ambigua) and the freshwater community is characterized by maximum values of both Cyperaceae and hydrophytic types, and by Poaceae. Subsequently, fossil data from a core from Mar Chiquita coastal lagoon were compared to the modern data set in search of similarities between fossil and modern pollen spectra. A composite DCA revealed that the fossil pollen assemblages have modern counterparts, permitting to interpret the vegetation history of the area in greater detail than previously possible. - 2003 Published by Elsevier B.V. Keywords: modern pollen; vegetation; environmental parameters gradient; Pampa grasslands; Holocene; vegetation history; Argentina
1. Introduction Along the coast of the Pampa grasslands, Ar-
* Corresponding author. Fax: +54-223-4753550. E-mail address: [email protected] (S. Stutz).
gentina, several paleoecological studies have demonstrated a complex history of Holocene vegetational and environmental changes in response to postglacial sea-level £uctuations, leading ultimately to modern patterns of plant communities (Prieto, 1996; Quattrocchio et al., 1998; Prieto et al., 1999; Stutz et al., 1999). Particularly, in the area of the Mar Chiquita coastal lagoon, changes
0034-6667 / 03 / $ ^ see front matter - 2003 Published by Elsevier B.V. doi:10.1016/S0034-6667(03)00084-8
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in plant communities have been reconstructed using pollen analysis by Nieto and D’Antoni (1985), Prieto (1993), Stutz (2000) and Stutz et al. (2002). Holocene sea-level £uctuations a¡ected the whole Argentine coastal zone, from Rio de La Plata to Southern Patagonia (Isla, 1989, 1998). Because of their own morphology, the nature and extent of sea-level £uctuations a¡ected every site in particular ways, and past changes in plant communities re£ect the local response to these £uctuations. The vegetation of the Mar Chiquita coastal lagoon area is characterized by several plant communities : psammophytic, halophytic and freshwater communities and isolated woodlands. These communities can be related to speci¢c environmental parameters, such as topography, water table depth, substrate type and salinity, which change along a few kilometers-long gradient from the Atlantic coast towards inland. The existence of well-marked landforms, such as beaches, coastal lagoons, salt marshes, barriers, and shallow lakes (lagunas) is related to the Holocene transgressive^regressive sea-level history (Schnack et al., 1982; Fasano, 1991; Violante, 1992). According to Janssen (1984), when local plant communities can be reconstructed in a spatial arrangement of pollen assemblages, and when these same groups are identi¢ed within a temporal (stratigraphic) arrangement of pollen groups in sediment cores, more accurate interpretations of the nature of local vegetation succession can be made. Therefore the spatial arrangement of plant communities in the Mar Chiquita coastal lagoon area provides an excellent potential for modern analogs that can be used to reconstruct the vegetational succession related to sea-level £uctuations. The principal aim of this paper is to establish the relationship between plant communities along a spatial gradient and their respective modern pollen assemblages, in order to obtain a more objective interpretation of fossil pollen assemblages. First pollen spectra of surface sediment samples are presented and numerical methods are used to classify them. Second, a numerical analysis is carried out to study analogies between modern and fossil pollen spectra.
2. Study area, physical setting and vegetation The study area is located in the southeastern part of the Pampa grasslands (Fig. 1). It has an extension of 300 km2 that encompasses the Mar Chiquita coastal lagoon (37‡43PS; 57‡24PW) and Laguna Hinojales (37‡34PS; 57‡27PW). The Mar Chiquita coastal lagoon has a north-south orientation and it is connected to the Atlantic Ocean by a narrow inlet, subject to natural and maninduced changes. It has an area of 46 km2 , with a maximum length of 25 km and a maximum width of 5 km. Laguna Hinojales has an area of 5 km2 and is located west of the Mar Chiquita coastal lagoon. The climate is subhumid^humid (Burgos and Vidal, 1951), with a mean annual temperature of 13.7‡C and a mean annual rainfall of 912 mm, mostly occurring in summer (Servicio Meteorolo¤gico Nacional, 1951^1980). A variety of landforms are present in the study area; some date back to late Pleistocene times, while others are related to the Holocene transgressive-regressive sea-level £uctuations or are still in evolution (Schnack et al., 1982; Fasano, 1991; Violante, 1992) (Fig. 1). Above the 5 m asl, the Pampa plain extends across the western and southern part of the study area, showing an extremely low topographic gradient. Its origin is related to dry conditions that prevailed during the late Pleistocene, shown by the presence of numerous shallow de£ation basins, today in many cases occupied by permanent freshwater (e.g. lagunas Hinojales and Nahuel Ruca¤). Bordering these depressions, half-moon shape silty dunes have developed reaching altitudes of 10^15 m above the surrounding plain (about 25 m asl). Their origin has been associated to the development of a de£ation^accumulation relief under arid conditions during the late Pleistocene. An elongated feature extending from near Laguna Nahuel Ruca¤ to the northeast and parallel to the coastline, separates the Pampa plain from landforms to the east. This Pleistocene barrier, with a maximum altitude of 10 m asl, is interpreted as an ancient sandy barrier related to the late Pleistocene marine transgression and together with the westernmost littoral shell ridge, mark the western and southern limit
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Fig. 1. Map showing the location of the Mar Chiquita coastal lagoon area, landforms, location of surface pollen samples and the site of La Lagunita core.
of the Holocene transgression (Fasano, 1991). Below 5 m asl, marginal £ats have developed ¢lled with Holocene sediments, that corresponds to the Mar Chiquita coastal lagoon and the surrounding salt marsh. South of Laguna de Sotelo, towards the north and also locally emerging within the marginal £at, there are littoral shell ridges. They constitute old
spits, bars and storm-beach accumulations formed during the Holocene transgressive^regressive sealevel £uctuations. The area is separated from the Atlantic Ocean by a coastal barrier, which corresponds to sandy dunes and adjacent beaches. This landform, with a maximum altitude of 30^40 m asl, extends from the Mar Chiquita lagoon inlet to the north increasing in height and width. The
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Fig. 2. Vegetation map of Mar Chiquita coastal lagoon area, modify after Isacch (2001). The location of surface pollen samples is shown.
coastal barrier represents the eolian cycle associated with the transgressive-regressive sea-level £uctuations, and continues in evolution today. Very distinctive plant communities develop on the above described landforms (Vervoorst, 1967; Stutz, 2001) (Fig. 2). Vegetation of the Pampa plain is characterized by a grass steppe dominated by grasses of the genera Piptochaetium, Stipa, Bo-
thriochloa and Aristida (Cabrera, 1976; Leo¤n, 1991). In more humid areas the vegetation is characterized by sedges (Cyperus, Carex, Schoenoplectus, Eleocharis), grasses (Paspalum, Stipa, Panicum, Polypogon, Glyceria, Paspalidium), several species of Juncus and herbaceous dicots (Eryngium, Aster, Daucus, Phyla). In the depressions, the shallow lakes plant communities (freshwater
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community) are characterized by aquatic plants (emergent, £oating-leaf and submergent). In the deepest zones the Myriophyllum elatinoides and Ceratophyllum demersum develop, while at the lake margins, Azolla ¢liculoides, Ricciocarpus natans and Lemna valdiviana are common. The dominant shore species is Schoenoplectus californicus, accompanied with Zizaniopsis bonariensis and Solanum glaucophyllum. Other common emergent species such as Senecio bonariensis, Ranunculus bonariensis, R. apiifolius, Alternanthera philloxeroides, Buddlegia elegans, Hydrocotyle bonariensis, H. ranunculoides, Polygonum punctatum, Eryngium sp. and other sedges such as Carex, Eleocharis and Cyperus are commonly found. On silty dunes, monospeci¢c woodlands of Celtis tala develop. Isolated individuals of Celtis tala also occur on the Pleistocene barrier and on the westernmost littoral shell ridge, as a consequence of bird seed dispersion. These woodlands constitute the southernmost distribution of the Tala district, which extends from the south of Entre R|¤os and Santa Fe provinces to the Mar Chiquita coastal lagoon as a narrow belt forming the eastern limit of Pampa grasslands (Cabrera, 1976). The vegetation of the Pampa plain is highly modi¢ed by farming activities that transformed the area into crop land and grazing ¢elds, mainly growing wheat, maize, soybean and arable pastures. On the marginal £at, the salt marsh of Mar Chiquita lagoon is characterized by a halophytic community (‘espartillar’) dominated by Spartina densi£ora. In the seasonally or permanently £ooded zone, a belt of Spartina densi£ora occurs bordering the lagoon accompanied by the mud£at colonist Salicornia ambigua. Above this zone, Distichlis spicata and D. scoparia codominate, accompanied by Atriplex montevidensis, Spartina alterni£ora, Grindelia discoidea, Sida leprosa, Limonium brasiliense, among the most important. In the highest zone, only a¡ected by episodic £ooding, patches of Juncus acutus occur. Also Paspalum vaginatum, Ambrosia tenuifolia, Hydrocotyle bonariensis, Scirpus cernuus, Apium sellowianum and Samolus valerandi are common. Bordering the freshwater canals that £ow into Chico creek or into the lagoon the sedges Schoenoplectus chilensis and Eleocharis bonariensis are frequent, as-
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sociated with other common freshwater species. The emergent littoral shell ridges have been transformed into grazing ¢elds and the natural vegetation is highly modi¢ed. The vegetation of the coastal barrier is mainly characterized by psammophytic species of Poaceae, Cyperaceae and Asteraceae families which grow on sandy soils (psammophytic community), and is distributed according to the morphology of the dunes. Near the beach and on recently formed dunes, patches of Spartina coarctata develop. Calycera crassifolia, Senecio crassi£orus and the adventitious Cakile maritima are the most important associated species. Mobile dunes are characterized by the pioneer grass Panicum racemosum. At the crests and slopes of low dunes as well as in lightly humid interdune depressions, Androtrichium tryginum and Tessaria absinthioides are dominant, frequently associated with Cortaderia dioica. In the interdune depressions where the water table is near the surface or outcropping, Typha angustifolia and T. latifolia, the sedges Schoenoplectus californicus, S. maritimus, Carex extensa, Eleocharis montevidensis and several species of Juncus prevail. On ¢xed dunes Adesmia incana and Poa lanuginosa are dominant, accompanied by Poa barrosiana, Hydrocotyle bonariensis, Margyricarpus pinnatus, Solidago chilensis, Oenothera mollissima, Polygala cyparissias, Senecio crassi£orus, Ambrosia tenuifolia, Baccharis juncea, B. microcephala, Gnaphalium cheiranthifolium, Daucus montevidensis, Androtrichium tryginum, and the adventitious Centaurium pulchellum, Blackstonia perfoliata, Medicago lupulina, Melilotus indicus and M. albus. Many exotic trees were introduced in the area at the beginning of the 20th century for public parks and gardens (e.g. Populus alba, Betula pendula, Casuarina sp., Juglans nigra, Araucaria araucana and A. angustifolia). On the coastal barrier plantations of Pinus, Cedrus, Cupressus, Eucalyptus and Acacia longifolia are common.
3. Materials and methods Sampling sites were chosen according to the major plant communities that develop on the different landforms. In the Pampa plain, because
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its vegetation is highly modi¢ed (Fig. 2), only shallow lakes freshwater communities were sampled. A total of 24 surface sediment samples were collected (Figs. 1 and 2). Seven samples from the freshwater community of Laguna Hinojales and 11 samples from the halophytic community of the salt marsh of Mar Chiquita lagoon were collected using a Dachnovsky core sampler and sub-sampling the top 2 cm of each core. Six samples were collected at the psammophytic community of the coastal barrier from the upper soil to a depth of 2^5 cm with a spatula. Sediment samples were processed according to standard palynological techniques including warm KOH, HCl, heavy-liquid separation with ZnCl2 , HF and acetolysis (Gray, 1965; Faegri and Iversen, 1989). Five Lycopodium clavatum tablets were added to every weighted sample before treatment to estimate representative pollen sums (Bianchi and D’Antoni, 1986). Total pollen sums varied from 230 to 1420 grains. Identi¢cations and counting were performed using the pollen reference collection of the Laboratory of Palaeoecology and Palynology, University of Mar del Plata. Additional pollen data from ¢ve surface sediment samples from the bottom of the Mar Chiquita coastal lagoon (Nieto, 1985) were considered. In the pollen diagrams some genera or species were grouped into family categories. Asteraceae pollen includes Ambrosia/Xantium, undi¡erentiated Asteroideae and Mutisieae. Apiaceae pollen includes Hydrocotyle, Apium, Eryngium, Daucus and undi¡erentiated Apiaceae. Other species were grouped according to growth habitat. Psammophytic types include herbs taxa that develop on sandy soils: Fagara sp., Adesmia incana, Calycera crassifolia, Oenothera mollissima, Blackstonia perfoliata, Centaurium pulchellum, Polygala cyparissias, Margyricarpus pinnatus and Caryophyllaceae; and hydrophytic types include emergent and submergent aquatic taxa: Alternanthera philoxeroides, Myriophyllum elatinoides, Ranunculus spp. and Solanum glaucophyllum. Extra-regional types and exotic arboreal taxa are not included in the pollen sums for percentage calculation. Pollen diagrams were plotted using TILIAGRAPH 1.25 (Grimm, 1991). Surface pollen samples were classi¢ed into
groups using Cluster Analysis (CA) and were ordinated by Detrended Correspondence Analysis (DCA; DECORANA). Statistical analyses were performed to determine whether the plant communities are associated with characteristic pollen spectra from which the parent vegetation can be identi¢ed. Only pollen taxa present at v 2% in at least three samples were included. Brassicaceae were excluded due to an overrepresentation in sample 4, which corresponds to the adventitious Cakile maritima, that would have distorted the statistical analyses. The analyses were performed on recalculated percentages after all modi¢cations were made. The ¢nal 12 pollen types selected were: Poaceae, Cyperaceae, Chenopodiaceae, psammophytic types, hydrophytic types, Asteraceae subf. Cichorioideae, Asteraceae, Celtis, Plantago, Typha, Papilionoideae and Apiaceae. Both analyses were performed using TILIA 1.12 (Grimm, 1991).
4. Results 4.1. Surface pollen diagram (Fig. 3) The zonation shown in this ¢gure is based on the classi¢cation of the sites at the time of sample collection (Figs. 1 and 2). The visual inspection of this ¢gure indicates the di¡erent plant communities/sites are distinguishable on the basis of their pollen assemblages. Samples from the psammophytic community of the coastal barrier (1^6) are characterized by variable values of Poaceae (12^68%) and Cyperaceae (6^57%) and maximum values of the psammophytic types (up to 6.5%). Samples from the halophytic community of the salt marsh (7^16) are characterized by maximum values of Chenopodiaceae (Salicornia ambigua) (55^80%) while the samples from the bottom of the Mar Chiquita coastal lagoon (17^22) are characterized by Chenopodiaceae (16^41%), Poaceae (10^34%), Asteraceae (14^30%) and Cyperaceae (6^16%). The samples from the freshwater community of Laguna Hinojales (23^29) are characterized by maximum values of both Cyperaceae (20^70%) and hydrophytic types (up to 5%), and by Poaceae (16^49%).
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PALBO 2539 18-9-03 Fig. 3. Percentage pollen diagram of 29 surface samples, arranged after sampling sites (plant community/landform).
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water community samples (cluster 3). Samples 15 and 16, also collected in the ‘espartillar’ but far from the lagoon’s shore, are grouped with coastal lagoon samples because of their low values of Chenopodiaceae. Samples 2 and 3 of the psammophytic community and sample 27 of the freshwater community are included in cluster 2 and became separated at the third level of division. These samples have the maximum values of Poaceae (up to 70%). The former two were collected near populations of Cortaderia dioica, and the latter was collected far from the Laguna Hinojales shore. Although in cluster 3, the samples of the psammophytic and the freshwater communities are mixed, at a third level of division the psammophytic community samples, except sample 1, are distinguished from the freshwater community samples, and at the fourth level the freshwater community samples which have maximum values of Cyperaceae (50^70%) and of hydrophytic types (up to 5%) (samples 23, 24, 28 and 29) became separated. 4.3. DCA (Fig. 5)
Fig. 4. CA of surface pollen samples. Samples are named according to sampling site: PS: psammophytic community; FW: freshwater community; HC: halophytic community and CL: coastal lagoon.
4.2. CA (Fig. 4) The ¢rst division clearly separates the most of the halophytic community samples (cluster 1) from all others. These samples have maximum values of Chenopodiaceae (55^80%) and were collected in the halophytic community (‘espartillar’) near the coast of Mar Chiquita lagoon, where Salicornia ambigua mainly develops. The second division separates all the coastal lagoon samples and samples 15 and 16 of de halophytic community (cluster 2), which have lower values of Chenopodiaceae (21^38%), from the most of both, psammophytic community samples and fresh-
The DCA ordination of the samples broadly equates with the plant communities/sites recognized in the ¢eld. The ¢rst axis (eigenvalue 0.35) contrasts Chenopodiaceae, Plantago and Asteraceae subf. Cichorioideae with Cyperaceae, psammophytic types, Apiaceae, Typha, hydrophytic types and Papilionoideae. It separates the halophytic community samples and the coastal lagoon samples from the psammophytic community samples and the freshwater community samples. Poaceae, Celtis and Asteraceae against Chenopodiaceae di¡erentiate only slightly between the coastal lagoon samples and the halophytic community samples. The second axis (eigenvalue 0.04) contrasts psammophytic types, Apiaceae, Celtis and Poaceae with hydrophytic types and Papilionoideae. It separates the psammophytic community samples from the freshwater community samples. Sample ordination suggests that along axis 1, plant communities change from freshwater to brackish conditions, re£ecting the increase of salinity in the substrate. Sites progressively separated along axis 2 re£ect the e¡ect of water table
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Fig. 5. DCA biplot of axes 1 and 2. (a) Sample scores with memberships of the respective sampling sites. (b) Pollen variable scores.
depth and substrate type in plant communities distribution.
5. Discussion When comparing modern pollen assemblages and vegetation, the three plant communities under consideration are easily distinguishable by their pollen spectra. Several interacting factors determine the degree in which each plant community is re£ected by the
pollen spectra. Among the most important are pollen production and dispersal characteristics of the species, the means of dispersion, insect or wind, the local-scale patterns of the species distribution at each community and the representativeness of local and extra-local pollen rain. Considering the three plant communities, the clearest signature is detected in the halophytic community re£ected by the samples from the salt marsh and from the coastal lagoon. These groups of samples are well discriminated in the CA and by the ¢rst axis of the DCA. The princi-
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pal pollen type associated with these groups is Chenopodiaceae. Samples from the halophytic community show an over-representation of this taxon (mainly representing Salicornia ambigua), even though the dominant species is Spartina densi£ora (Poaceae). This is probably attributable to a di¡erence in their pollen dispersion^deposition. Both species are abundant pollen producers and wind-pollinated. Spartina bears its in£orescence to 1^1.5 m instead of Salicornia that only can reach 0.80 m high. This di¡erence in height probably limits the uplift of Chenopodiaceae pollen into the turbulent upper air layer, greatly restricting its pollen dispersion and favoring in situ deposition. Samples 15 and 16 were grouped with the samples from the coastal lagoon. They were collected in the middle zone of the salt marsh, far from the coast of the lagoon where Salicorna ambigua declines in number and the co-dominant species are Distichlis spicata, D. scoparia and Spartina alterni£ora (Poaceae). Chenopodiaceae is well represented when present locally, but pollen values decline rapidly with distance from the source, or when its presence is only a small proportion of the vegetation. Samples from the coastal lagoon, mainly characterized by Chenopodiaceae, Poaceae and Asteraceae, also represent the halophytic community. Chenopodiaceae percentages are less than 50% and similar to those of Poaceae. These samples form a tight group in the CA and became slightly separated from the halophytic community samples by the ¢rst axis of the DCA. In addition to Chenopodiaceae pollen taxa associated with these samples are Poaceae, Plantago, Celtis, Asteraceae, and Asteraceae subf. Cichorioideae, emphasizing the role of the lagoon as a pollen catchment site. The lagoon not only collects pollen from the surrounding low salt marsh, but also pollen from species of the middle and the high salt marsh, such as Distichlis and Juncus acutus (Monocotyledonae). On the contrary, samples from psammophytic and freshwater communities show some overlap in pollen assemblages, as indicated by the ¢rst axis of the DCA, the CA groups and the pollen percentages diagram, where both groups of samples are similar because their comparable Cyperaceae and Poaceae values. Due to the di⁄culty of
a genus or species level identi¢cation, both Poaceae and Cyperaceae include psammophytic species (Androtrichium tryginum, Schoenoplectus maritimus, Cortaderia dioica, Poa lanuginosa, P. barrosiana, Spartina coarctata and Panicum racemosum) as well as aquatic species (Schoenoplectus californicus, Carex sp., Eleocharis sp., Cyperus sp. and Zizaniopsis bonariensis). Another point to be considered is that both plant communities share some species, like the sedges that also occur at interdune depressions (Schoenoplectus, Carex and Eleocharis). The separation between these communities is possible by few taxa poorly represented in the pollen spectra : the speci¢c psammophytic and hydrophytic types, as it is shown in the second axis of the DCA. Under-representation of these types in the pollen spectra is due to their low plant frequency and that they are insect-pollinated producing relatively low quantities of pollen grains. Their importance for the characterization of these communities is clearly shown by the second axis of DCA, where they are correlated with both, the substrate type and water table depth. Aquatic species included in hydrophytic types develop on the silty-clay substrates of the shallow lakes, where the water table is near the surface or outcropping, while psammophytic species, included in psammophytic types, develop on the sandy substrates of the dunes, where the water table is deeper.
6. Application to interpret fossil pollen assemblages To illustrate the applicability of the modern pollen assemblages in the interpretation of plant community evolution, 41 fossil samples from La Lagunita core (Figs. 1 and 6) were combined with the 29 modern surface samples using DCA (Fig. 7). The fossil pollen diagram was divided in three zones and two subzones by CA Stratigraphical Constrained (Stutz, 2000). Zone LL1 (ca. 6200^ 5130 yr BP) is dominated by Poaceae accompanied by Chenopodiaceae and presents constant values of psammophytic types. Zone LL2 (5130^ 2570 yr BP) is characterized by an increase of Chenopodiaceae and a decrease of Poaceae. At
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Fig. 6. Pollen percentage diagram of La Lagunita core. Pollen zones according to CA, depth, radiocarbon dates and estimated ages are indicated.
the top of this zone (LL2b) psammophytic types decrease. Zone LL3 (2570 yr BP-present) is characterized by the highest values of Chenopodiaceae. DCA ordination of modern and fossil samples shows that except for the pre-5130 yr BP interval, the fossil samples have modern counterparts, thus the gradual transition and replacement of plant communities can be interpreted on the basis of modern plant communities. Zone LL1 has no analogs in the modern spectra. The relatively high Poaceae values combined with high Chenopodiaceae suggests a mosaic of psammophytic and halophytic communities developing on sandy barrier and salt marsh/coastal lagoon
environment that have no modern counterpart. Subzones LL2a and LL2b form a continuum that is partially analogous to coastal lagoon and salt marsh halophytic communities. The whole zone LL2 shows the gradual replacement from the psammophytic^halophytic mosaic to the halophytic community, which increases its development until the modern state, as shown by zone LL3. Zone LL3 falls within the halophytic salt marsh community group, suggesting that the installation of this community began ca. 2500 yr BP. The continuous presence of Celtis tala in the fossil spectra, with percentage values similar to those of the modern spectra, suggests that these
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Fig. 7. DCA of combined surface and fossil pollen samples.
woodlands have been present in the area since Middle Holocene.
7. Conclusions Di¡erent plant communities in Mar Chiquita coastal lagoon area are clearly distinguished by their pollen spectra. These communities, determined by environmental parameters such as topography, salinity, substrate type and water table depth, re£ect the few kilometers-long gradient from coast to inland. Furthermore, the precision with which modern pollen spectra can de¢ne plant community boundaries at a small spatial scale, greatly increases the accuracy in the reconstruction of past vegetation. Such precision is desirable for paleoecological reconstructions dealing with changes in local plant communities related to sea-level £uctuations. In addition, this study provided valuable analogs to interpret Holocene vegetational history (e.g. Stutz, 2000; Stutz et al., 2002), related to sea-level £uctuations in other sites on the coast of the Pampa grasslands.
Acknowledgements This research was funded through grants from
the Universidad Nacional de Mar del Plata (15/ E138), CONICET (PIP N‡0418/98) and Agencia Nacional de Promocio¤n Cient|¤¢ca y Tecnolo¤gica FONCYT (PICT 07-06477). Thanks are due to V. Mancini and F.I. Isla for their helpful comments; Claudio Perez for his help in the ¢eld and for improvement of the English text; R and M. Arbelaiz and A. Romano for giving permission to work in their properties ; M. Tonello for drafting Fig. 1. We are grateful to H. Hooghiemstra and to an anonymous reviewer for critical and valuable suggestions. References Bianchi, M.M., D’Antoni, H.L., 1986. Depositacio¤n del polen actual en los alrededores de Sierra de Los Padres (Pcia. de Buenos Aires). IV Congreso Argentino de Paleontolog|¤a y Bioestratigraf|¤a. Ape¤ndice de las Actas, 16^27. Mendoza, Argentina. Burgos, J., Vidal, A.L., 1951. Los climas de la Repu¤blica Argentina segu¤n la nueva clasi¢cacio¤n de Thornthwaite. Meteoros 1, 1^32. Cabrera, A.L., 1976. Regiones Fitogeogra¤¢cas Argentinas. Enciclopedia Argentina de Agricultura y Jardiner|¤a. Editorial ACME. T.II. Buenos Aires. 85 pp. Faegri, K., Iversen, J., 1989. Texbook of pollen analysis. In: Faegri, K., Kalana, P.E. and Krzywinski, K. (Eds.), IV Edition. John Willey and Sons, New York, 328 pp. Fasano, J.L., 1991. Geolog|¤a y geomorfolog|¤a. Regio¤n III. Faro Querand|¤ - Mar de Cobo. Provincia de Buenos Aires.
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